It is often desired to construct a cross-sectional view (layer or slice) and/or three-dimensional (3D) view of an object for which actually presenting such views is difficult or impossible, such as due to irreparably damaging the object. For example, imaging systems are utilized in the medical arts to provide a view of a slice through a living human's body and to provide 3D views of organs therein. Similarly, imaging systems are utilized in the manufacture and inspection of industrial products, such as electronic circuit boards and/or components, to provide layer views and 3D views for inspection thereof.
Often, desired images are provided through reconstruction techniques which use multiple two-dimensional (2D) radiographic, e.g., X-band radiation (X-ray), images, e.g., detector images. The technique of reconstructing a desired image or view of an object (be it a 3D image, a cross-sectional image, and/or the like) from multiple projections (e.g., different detector images) is broadly referred to as tomography. When such reconstructing of a cross-sectional image is performed with the aid of a processor-based device (or “computer”), the technique is broadly referred to as computed (or computerized) tomography (CT). In a typical example application, a radiation source projects X-band radiation through an object onto an electronic sensor (or “detector”) array thereby providing a detector image. By providing relative movement between one or more of the object, the source, and the sensor array, multiple views (multiple detector images having different perspectives) may be obtained. An image of a slice through the object or a 3D image of the object may then be approximated by use of proper mathematical transforms of the multiple views. That is, cross-sectional images of an object may be reconstructed, and in certain applications such cross-sectional images may be combined to form a 3D image of the object.
Within X-ray absorption tomography, a number of imaging techniques are applicable to reconstruction of cross-sectional slices. One imaging technique is known as laminography. In laminography, the X-ray source and sensor are moved in a coordinated fashion relative to the object to be viewed so that portions of an object outside a selected focal plane lead to a blurred image at the detector array. Focal plane images are reconstructed in an analog averaging process. Examples of laminography systems that may be utilized for electronics inspection are disclosed in U.S. Pat. No. 4,926,452 entitled “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS” and in U.S. Pat. No. 6,201,850 entitled “ENHANCED THICKNESS CALIBRATION AND SHADING CORRECTION FOR AUTOMATIC X-RAY INSPECTION.” An advantage of laminography is that extensive computer processing of ray equations is not required for image reconstruction.
Another imaging technique is known as tomosynthesis. Tomosynthesis is an approximation to laminography in which multiple projections (or views) are acquired and combined. As the number of views increases, the resulting combined image generally approaches that obtained using laminography with the same geometry. A differentiating feature of tomosynthesis from the above-described laminographic technique is that in tomosynthesis X-ray images obtained from different directions (different angles of view) can be manipulated (e.g., overlapped with different spatial shifts and their brightness averaged) to produce a variety of cross-sections. In other words, one set of X-ray images can be used to obtain multiple cross-sections of an object under inspection (e.g., cross-sections of the object at different heights). Tomosynthesis may be performed as an analog method, for example, by superimposing sheets of exposed film. Tomosynthesis may, instead, be performed as a digital method. In digital tomosynthesis, the individual views are divided into pixels, and digitized and combined via computer software.
Tomosynthesis is of particular interest for use in relatively high-speed applications in which images are desired to be reconstructed very quickly, e.g., in real-time. Examples of such high-speed applications include, without limitation, reconstructing images for medical applications and reconstructing images for automated inspection of industrial products. For instance, as medical procedures continue to evolve, certain medical applications are beginning to desire fast reconstruction of cross-sectional images. Real-time X-ray imaging is increasingly being desired by medical procedures, such as many electro-physiologic cardiac procedures, peripheral vascular procedures, percutaneous transluminal catheter angioplasty (PTCA) procedures, urological procedures, and orthopedic procedures, as examples. Additionally, reconstruction of cross-sectional images from radiographic (e.g., X-ray) images has been utilized in quality control inspection systems for inspecting a manufactured product, such as electronic devices (e.g., printed circuit boards). That is, tomosynthesis may be used in an automated inspection system to reconstruct images of one or more planes (which may be referred to herein as “depth layers” or “cross-sections”) of an object under study in order to evaluate the quality of the object (or portion thereof). An X-ray imaging system may capture detector images (e.g., pixels) of a circuit board at various locations and at various orientations. Primarily, one is interested in images that lie in the same plane as the circuit board. In order to obtain these images at a given region of interest, raw X-ray detector images (pixels) may be mathematically processed using a reconstruction algorithm (e.g., Backprojection or shift-and-add algorithms) to reconstruct cross-sectional layers or slices.
For instance, a printed circuit board (or other object under study) may comprise various depth layers of interest for inspection. As a relatively simple example, a dual-sided printed circuit board may comprise solder joints on both sides of the board. Thus, each side of the circuit board on which the solder joints are arranged may comprise a separate layer of the board. Further, the circuit board may comprise surface mounts (e.g., a ball grid array of solder) on each of its sides, thus resulting in further layers of the board. The circuit board (or other object under study) may be imaged from various different angles of view (e.g., by exposure to X-rays at various different angles) resulting in radiographic images of the circuit board (e.g., pixel data), and such radiographic images may be processed to reconstruct an image of a layer (or “slice”) of the circuit board. Thereafter, the resulting cross-sectional images may, in some inspection systems, be displayed layer by layer, and/or such cross-sectional images may be used to reconstruct a full 3D visualization of the object under inspection.
In laminography, only one layer may be reconstructed at a time. A potential advantage of tomosynthesis is that many different layers may be reconstructed from a given set of projection (detector) image data. Thus, the potential for more efficient reconstruction exists with tomosynthesis because a new set of image data need not be acquired for every layer of an area of an object to be reconstructed. However, while tomosynthesis allows for many different layers to be reconstructed from a given set of image data, only a few of those layers may be of interest, such as those corresponding to the top and bottom surfaces of a circuit board under inspection. The location of those layers of interest may be obtained in advance, as must be done in laminography, using an appropriate locating system, or, for tomosynthesis, may be done after data acquisition using an appropriate analysis of image layers. In the latter case, the selected image may be one that maximizes some constraint, such as image sharpness. When this analysis is automated using a processing unit, e.g. a digital computer, it is broadly referred to as “auto-focusing.”